a Portable Cofferdam. 145 



the water, were guided to the spot by the men in charge, 

 and whenever they were in the desired position, the men at 

 once moored or fixed the frames to the bottom, by driving 

 down a pile at each corner. After this was done, all the piles 

 were placed between the frames and driven down, and keyed 

 up by the small piles called " closers." Four iron jumpers 

 J, were then driven down to their proper places outside of 

 the frames, and edge planks for retaining the clay were 

 slipped down upon the jumpers through ii'on staples, which 

 were fixed to the planks. After this good clay (which should 

 have some gravel mixed with it, to protect it from the wash 

 of the sea), was punned hard between the planks and the 

 cofferdam, after which the mast N was erected, and the 

 water taken out by means of the iron scoop shewn in the 

 drawing, which not only was used in taking out the stuff", but 

 proved far more efficacious than any pump we ever had. 

 Indeed, to get the dam pumped dry was for long the greatest 

 difficulty we had to contend with. But Mr William Downie, 

 to whom I gave the charge, soon removed this difficulty, by 



^p. The capacity of the 

 jhey generally made nine 

 und this method greatly 



• piles were from time to 

 .sing tide began to come 

 the clay, the men, before 

 ing or " deck,^' as it was 

 ads of the planks resting 

 upon the top of the inner frame. Un this deck, ballast (con- 

 sisting of stones of a convenient size) was deposited to prevent 

 the whole frame from being floated up, — the quantity so de- 

 posited varying with the height of tide, or appearance of the 

 weather. As each compartment of the excavation was com- 

 pleted, and before the dam was removed, the rock below the 

 two rows of piles which adjoined the next cuttings was com- 

 pletely taken out, and the piles driven down to the bottom of 

 the excavated pit, and left standing.* When the dam was 



* Before lifting the cofferdam, the pit was filled with sand, to support the 

 VOL. XLV. NO. LXXXIX. — JULY 1848. K 



